The invention relates to the field of radio frequency (RF) devices and semiconductor manufacturing processes. More particularly, the present invention relates to RF switches using semiconductors microelectromechanical systems (MEMS) and semiconductor manufacturing processes.
Radio frequency (RF) devices are commonly used in communication systems where high frequency operation is required. One device used in communication systems is an RF switch that is a mechanical switch switching at high-speed for use in RF communication systems. Microelectromechanical systems (MEMS) are miniature devices that are being manufactured in a wide variety of mechanical forms. MEMS devices are inherently both mechanical and electrical devices that are subject to wear and contamination and suffer from limited life times. Electrical functionality is often limited by the mechanical durability of the MEMS devices. RF MEMS switches offer high-speed operation for RF communication systems but suffer from speed limitations inherent in mechanical systems.
U.S. Pat. No. 5,578,976 issued Nov. 26, 1996 discloses an RF MEMS switch device. This switch device has a suspended arm that is attached on one side to a substrate and provides a conductive pad on another freely suspended side using a cantilever arm that extends over a ground line. The device is subject to contamination. The freely suspended cantilever arm suffers from an inherent mechanical weakness by virtue of flexing back and forth the cantilever arm at a single connection point. The switch device can be used as an AC capacitive coupler for communicating an RF signal across DC biased contacts on the cantilever arm and the supporting substrate. The RF MEMS switch with a suitable DC bias can also function as a DC coupled RF switch.
U.S. Pat. No. 5,638,946 issued Jun. 12, 1997 also discloses an RF MEMS switch and also discloses a suspended arm that is attached on one side to a substrate as a cantilever arm suffering from a single point of flexing wear and stresses on the attached side of the arm. This RF MEMS switch also suffers from contamination and limited lifetime. The RF MEMS device is suitable as a direct DC switch coupler or as an AC coupler with a limited operation frequency range.
These RF MEMS switches, though intended to operate at high switching speed, are limited in speed of actuation due to the inherent nature of the extended cantilever arm that must substantially flex up and down during operation over the electrical contacts and waveguides. The RF MEMS switches suffer from contamination due to exposure of debris formed during both manufacture and operational use. The asymmetric suspension mechanical configuration functions as an uncontrolled one ended suspension spring, providing uncontrolled mechanical oscillations during use, disadvantageously effecting the electrical performance of the RF switch. The RF switches are made larger than that minimally required due to the suspension cantilever arm, due to the use as a mechanical spring return, and due to the substrate pad placement being extended to the end of the cantilever arm. The physical arrangement of the RF switch electrodes significantly deviates from an ideal RF transmission line and consequently perturbs the propagation of RF signals due to impedance mismatch. Additionally, because of the inherent rotational operation of the contact end of the cantilever arm, the RF switch exhibits an asymmetric electrical performance as the arm rotationally flexes during operation when the contact pads are not consistently aligned with the substrate contact pads. The contact pads, when in contact with each other, suffer from stiction that slows the speed of operation and limits the effective operating range of the MEMS RF switches. These and other disadvantages are solved or reduced using the present invention.
An object of the invention is to provide a microelectromechanical systems (MEMS) radio frequency (RF) switch for high-speed electrical operation.
Another object of the invention is to provide a MEMS RF switch that is resistant to external contamination during use.
Yet another object of the invention is to provide a MEMS RF switch that can be controlled by opposing DC bias voltages for controlled electrical operation.
Still another object of the invention is to provide a MEMS RF switch having equilaterally suspended contacts for evenly distributed flexing and wear during operational use.
A further object of the invention is to provide a MEMS RF switch that is resistant to contamination through MEMS encapsulation of the operational contact pads of the MEMS RF switch during manufacture.
Another object of the invention is to provide a MEMS RF switch that has contact pads subject to both pull up and pull down biasing for controlled electrical operation.
Still another object of the invention is to provide a MEMS RF switch that can be operated as an RF AC coupler during operational use.
Still another object of the invention is to provide a MEMS RF switch that can be operated with direct con tact symmetric coupling during operational use.
Yet another object of the invention is to provide a MEMS RF switch having distortion free RF operation by virtue of equilateral coupling suspension with uniform signal propagation along an uninterrupted RF transmission.
The invention is directed to a MEMS RF switch that is optimized for operation over 1 GHz. The device has a vertical contact pad alignment configuration of electrodes and transmission lines such that minimal RF distortion, loss, and reflections will be created in the switch. Distortion free operation is accomplished by the use of a continuous grounded coplanar transmission line structure for the RF transmission line through the MEMS switch structure. The switch has on and off transition times that are symmetric and perfected by electrostatic actuation in both up and down directions. The vertical alignment configuration of electrodes contacts and waveguides have a minimal area switch contact with minimum moving mass during electrostatic actuation. The actuation electrodes are suspended above the transmission line and move up and down during minimal supporting spring forces during electrostatic actuation. The switch mechanical design provides rapid ON and OFF switching times. The switching speed is primarily a function of the inertia of the rest mass and switching potential of the electrostatic potentials that can further function to restore the switch to the ON or OFF conditions.
The RF switch is a MEMS device for switching signals through an RF transmission line. The RF MEMS switch is enclosed using two opposing substrates bonded together. A MEMS electrode substrate and an RF transmission line substrate are firstly separately manufactured and then bonded together to encapsulate, that is, entomb the composite RF MEMS switch. The opposing substrate switch design enables independent fabrication and process optimization of both the MEMS switch portion and RF transmission line portion of the composite RF MEMS switch. The composite substrate configuration of the RF MEMS switch increases manufacturing yields with improved performance. The RF MEMS switch is fabricated by wafer bonding the MEMS switch substrate in vertical alignment with the RF transmission line substrate so that electrostatic electrodes and switch coupler of the MEMS switch substrate are in respective vertical alignment with electrostatic electrodes and RF transmission lines of the RF transmission line substrate. The opposing substrate bonding process enables the RF transmission wafer and MEMS switch wafer, when bonded together, to be hermetically sealed from ambient dirt and contamination by encapsulation further increasing switch yield and long term switch reliability. After wafer bonding, the composite wafers maybe further processed using conventional packaging and wafer-sawing methods without risk of contaminating the delicate released MEMS structures. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.